Colour matching on press

ImageHow can we use a colour measurement device to determine if a particular colour is an acceptable match? We all know that colour is extremely subjective and can look different when viewed under various lighting conditions. The good news is that we have a method to measure colour differences.

In many print shops you are given a proof or a previously printed sample and told to match it. Using the method outlined below we can produce a printed sample, measure it with our spectrophotometer and determine if the match is acceptable or if it requires more work before showing it to the customer.

The first thing we need to understand and probably already know is that what my eyes and your eyes see can be very different. To solve this problem we need to measure colour using a simple device that is not emotionally involved in the project. A spectrophotometer is the best tool to accomplish this task. These devices come in two flavours: a handheld, self-contained unit or a tethered unit that is connected to a computer via a USB connection. Both work equally well but the non-tethered unit is going to be more versatile in most print environments.

Properly making use of this device requires the understanding of a term called delta E (dE). Delta E, is a single number that represents the ‘distance’ between two colours. The idea is that a dE of 1.0 is the smallest colour difference the human eye can see. A dE less than 1.0 is considered imperceptible and it stands to reason that any dE greater than 1.0 is noticeable.

The measured number is used to determine how closely the two colours resemble or match each other. The lower the number the closer the match. As we learned, a value between 0-1 is considered a perfect match as our visual system cannot see any difference this small. Depending on your industry you will have varying differences or tolerances as to what is considered an acceptable match.

Without getting overly technical we need to understand that there are a few flavours of dE and each will interpret your measurements slightly differently.

Delta-Lab and Delta-LCH, CMC 2:1, dE94 & dE2000 are the main five flavours of dE. The ones that you need to be the most concerned with are: dE2000 (an updated version of dE94), CMC and Lab. On my trusted X-Rite 530 I default to CMC.

Delta E Measurement Guide:
0-1  Is considered a perfect match. Our eyes cannot see a colour difference less than a 1 Delta E. If you hit this range you can go home and put your feet up.

1-3 This would be considered a very acceptable match in the printing and prepress environment, but too tight a tolerance for most sign/large format printers.

4-6 This would be getting outside an acceptable match for most lithographic print but acceptable for many large or grand format sign printers.

6   If you are into this category it is time to put on a pot of coffee because you are going to be around for a while more.

The reason that the tolerances are higher for the sign market is due to the reduced colour range available on many of the solvent and pigment ink-sets seen on these devices.

In the end measurements will help to support your belief that a colour is a close match but it is our eyes that have to be sold that a colour matches. dE is a starting point and there are instances where a colour will appear to be out of tolerance and still show as being an acceptable match in dE. Again use dE as a supporting tool but in the end confirm all measurements in a light booth with your eyes.

Next month we are going to look at how you can utilize a software program called Eye-One Share freeware to evaluate and compare colours.    

Angus Pady is the president of Digital Solutions. Complete colour control from desktop to press. T: 905-764-6003